The rapid development of mobile information terminals such as cellphones, laptops, and smartphones into smaller and lighter ones in recent years has led to a need for higher-capacity secondary batteries as power supplies for driving them. Nonaqueous electrolyte secondary batteries, which charge and discharge through the movement of lithium ions between positive and negative electrodes in association with charging and discharging, are widely used as power supplies to drive such mobile information terminals because of their high energy density and high capacity.
More recently, nonaqueous electrolyte secondary batteries have been focused on as power supplies for the operation of electric tools, electric vehicles (EVs), and hybrid electric vehicles (HEVs and PHEVs) and are expected to be used in a broader range of fields. Such a power supply for machine operation needs to have an increased capacity that allows for extended use and improved output characteristics for repeated high-rate charge and discharge in a relatively short period. In particular, in applications such as electric tools, EVs, HEVs, and PHEVs, it is essential to achieve a high capacity while maintaining output characteristics during high-rate charge and discharge.
Possible methods for increasing the capacity of a nonaqueous electrolyte secondary battery include the use of a material with a high Ni content of a positive electrode active material and increasing the charging voltage. However, positive electrode active materials the Ni content of which is high have the problem of increased resistance after cycling, although having high capacities.
For example, PTL 1 below suggests that allowing an element of Group 3 in the periodic table to be present on the surfaces of matrix particles as a positive electrode active material prevents the positive electrode active material from being reacting with the electrolytic solution and mitigates the associated degradation of charge and storage characteristics even at increased charging voltage.
Furthermore, PTL 2 below suggests that coating a core that contains a lithium compound with two or more surface treatment layers that contain compounds of coating elements provides a positive electrode active material for lithium secondary batteries that has superior capacity, electric energy, and cycle-life characteristics.